1. Field of the Invention
The present invention relates generally to stereolithography and, more specifically, to the use of stereolithography in forming multilayer solid structures with vertical or near-vertical sides, such structures including packages for semiconductor devices and other electronic components and assemblies thereof
2. State of the Art
In the past decade, a manufacturing technique termed xe2x80x9cstereolithographyxe2x80x9d (STL), also known as xe2x80x9clayered manufacturing,xe2x80x9d has evolved to a degree where it is employed in many industries.
Essentially STL, as conventionally practiced, involves utilizing a computer to generate a three-dimensional (3-D) mathematical simulation or model of an object to be fabricated, such generation usually being effected with 3-D computer-aided design (CAD) software. The model or simulation is mathematically separated or xe2x80x9cslicedxe2x80x9d into a large number of relatively thin, parallel, usually vertically superimposed layers, each layer having defined boundaries and other features associated with the model (and thus the actual object to be fabricated) at the level of that layer within the exterior boundaries of the object. A complete assembly or stack of all of the layers defines the entire object, and surface resolution of the object is, in part, dependent upon the thickness of the layers.
The mathematical simulation or model is then employed to generate an actual object by building the object, layer by superimposed layer. A wide variety of approaches to stereolithography by different companies has resulted in techniques for fabrication of objects from both metallic and nonmetallic materials. Regardless of the material employed to fabricate an object, stereolithographic techniques usually involve disposition of a layer of unconsolidated or unfixed material corresponding to each layer within the object boundaries, followed by selective consolidation or fixation of the material to at least a semisolid state in those areas of a given layer corresponding to portions of the object, the consolidated or fixed material also at that time being substantially concurrently bonded to a lower layer. The unconsolidated material employed to build an object may be supplied in particulate or liquid form, and the material itself may be consolidated or fixed or a separate binder material may be employed to bond material particles to one another and to those of a previously formed layer. In some instances, thin sheets of material may be superimposed to build an object, each sheet being fixed to a next lower sheet and unwanted portions of each sheet removed, a stack of such sheets defining the completed object. When particulate materials are employed, resolution of object surfaces is highly dependent upon particle size, whereas when a liquid is employed, surface resolution is highly dependent upon the minimum surface area of the liquid which can be fixed and the minimum thickness of a layer which can be generated. Of course, in either case, resolution and accuracy of object reproduction from the CAD file is also dependent upon the ability of the apparatus used to fix the material to precisely track the mathematical instructions indicating solid areas and boundaries for each layer of material. Toward that end, and depending upon the layer being fixed, various fixation approaches have been employed, including particle bombardment (electron beams), disposing a binder or other fixative (such as by ink-jet printing techniques), or irradiation using heat or specific wavelength ranges such as found in a generated laser beam.
An early application of stereolithography was to enable rapid fabrication of molds and prototypes of objects from CAD files. Thus, either male or female forms on which mold material might be disposed might be rapidly generated. Prototypes of objects might be built to verify the accuracy of the CAD file defining the object and to detect any design deficiencies and possible fabrication problems before a design was committed to large-scale production.
In more recent years, stereolithography has been employed to develop and refine object designs in relatively inexpensive materials and has also been used to fabricate small quantities of objects where the cost of conventional fabrication techniques is prohibitive for same, such as in the case of plastic objects conventionally formed by injection molding. It is also known to employ stereolithography in the custom fabrication of products generally built in small quantities or where a product design is rendered only once. Finally, it has been appreciated in some industries that stereolithography provides a capability to fabricate products, such as those including closed interior chambers or convoluted passageways, which cannot be fabricated satisfactorily using conventional manufacturing techniques.
To the inventors"" knowledge, stereolithography has yet to be applied to mass production of articles in volumes of thousands or millions, or employed to produce, augment or enhance products including other, pre-existing components in large quantities, where minute component sizes are involved, and where extremely high resolution and a high degree of reproducibility of results is required. Furthermore, conventional stereolithography apparatus and methods fail to address the difficulties of precisely locating and orienting a number of pre-existing components for stereolithographic application of material thereto without the use of mechanical alignment techniques or to otherwise assure precise, repeatable placement of components.
In the electronics industry, state-of-the-art packaging of semiconductor dice is an extremely capital-intensive proposition. In many cases, discrete semiconductor dice carried on, and electrically connected to, lead frames are individually packaged with a filled-polymer material in a transfer molding process. A transfer molding apparatus is extremely expensive, costing at least hundreds of thousands of dollars in addition to the multihundred thousand dollar cost of the actual transfer molding dies in which strips of lead frames bearing semiconductor dice are disposed for encapsulation.
Further, encapsulative packaging of a semiconductor device already mounted on a substrate by molding and other presently used methods may be very difficult, time-consuming and costly. In some cases, the device may be packaged using a so-called xe2x80x9cglob-topxe2x80x9d of a material such as silicone, but the resulting seal is usually nonhermetic, the technique requires either substantial area around a die or a dam structure to contain the gel-like material, and the xe2x80x9cpackagexe2x80x9d boundary is not well-defined.
Use of stereolithography for packaging of semiconductor device components has been suggested. See, for example, copending U.S. patent applications Ser. Nos. 09/259,142 and 09/259,143, each filed on Feb. 26, 1999, pending, and the disclosures of each of which are hereby incorporated herein by this reference. While the feasibility and effectiveness of such a packaging technique has been proven, it has been recognized by the inventors herein that it would be desirable to achieve a surface finish on stereolithographically formed packaging and other structures which is superior to that presently obtainable.
The present invention provides an improvement to a stereolithography (STL) apparatus for forming a precisely dimensioned and finished miniature structure from a liquid or semiliquid photopolymer material by precise scanning of a beam of polymerization stimulating laser light. The structure is created by forming one or more layers of at least partially hardened or semisolid material, in which each subsequently formed layer is at least partially overlying and attached to a previously formed layer.
While the method and apparatus of the invention may be broadly applied to the fabrication of either a freeform structure or a structure attached to another object or objects, it will be exemplified herein as applied to the packaging of small items such as electronic components and, specifically, semiconductor dice. For example, a semiconductor die may be provided by this invention with a protective structure in the form of a layer of dielectric material having a controlled thickness or depth over an upper or lower surface and stacked layers located adjacent to lateral or peripheral die surfaces.
As used herein, the term xe2x80x9cpackagexe2x80x9d as employed with reference to electrical components includes partial as well as full covering of a given semiconductor die or other electronic component surface with a dielectric material and specifically includes, without limitation, partial and full covering of a bare semiconductor die as well as a semiconductor die previously configured as a so-called xe2x80x9cchip scalexe2x80x9d package, wherein the package itself, including the die, is of substantially the same dimensions as, or only slightly larger than, the die itself.
The packaging method of the present invention may be applied, by way of example and not limitation, to a die mounted to a lead frame (having a die mounting paddle or in a paddleless leads-over-chip (LOC), or in a leads-under-chip (LUC) configuration), mounted to a carrier substrate in a chip-on-board (COB) or board-on-chip (BOC) arrangement, to flip-chip configured semiconductor dice, or in other packaging designs, as desired.
The present invention employs computer-controlled, 3-D CAD initiated, stereolithographic techniques to apply protective and alignment structures to an electronic component such as a semiconductor die. A dielectric layer or layer segments thereof may be formed over or adjacent a single die or substantially simultaneously over or adjacent a large number of dice or die locations on a semiconductor wafer or other large-scale semiconductor substrate, individual dice or groups of dice then being singulated therefrom. The package may be formed, after singulation of the die from a wafer, to cover the lateral surfaces as well as the upper and/or lower surfaces of the die.
Precise mechanical alignment of singulated semiconductor dice or larger semiconductor substrates having multiple die locations is not required to practice the method of the present invention, which includes the use of machine vision to locate dice and features or other components thereon or associated therewith (such as lead frames, bond wires, solder bumps, fiducial marks, etc.) as well as features on a larger, carrier substrate for alignment and material disposition purposes.
In a presently preferred embodiment of the invention, the object or structure is fabricated using precisely focused electromagnetic radiation in the form of an ultraviolet (UV) wavelength laser under control of a computer and responsive to input from a machine vision system such as a pattern recognition system to fix or cure a liquid material in the form of a photopolymer.
A multilevel package structure is formed for example, by partially submerging an object such as a semiconductor die in a bath of liquid photopolymer material, the latter forming a thin layer comprising the lowermost portion of the package structure. A generally vertical laser beam of coherent radiation is controllably scanned over selected portions of the thin layer of photopolymer material for partial polymerization thereof, forming a self-supporting rigid or semirigid layer.
The object is then lowered to form a second thin layer of liquid photopolymer material over the prior, partially polymerized layer, followed by laser exposure. A stack of partially polymerized layers may thus be serially formed, comprising as many consecutive layers as are required to achieve the desired structure height.
Conventional laser systems used in STL equipment are capable of providing a focused laser beam xe2x80x9cspotxe2x80x9d of the desired diameter for die packaging, i.e., about 0.002-0.008 inch. While it is considered desirable to generate a laser beam which in scanning forms a spot with uniform power exposure or density throughout the spot, the power is nevertheless typically concentrated in the middle, or central, portion of the beam spot. Thus, as the polymerization-stimulating laser beam is scanned over a photopolymer material, the effective exposure and resulting initiation and stimulation of polymerization of the photopolymer will be greatest in a central region of the scanning path, with a rapid fall-off to either side. As a result, the exterior surfaces of a layered photopolymer structure""s lateral walls are not immediately polymerized to a desired degree, and interstitial crevices are formed at the joints between adjacent layers. The resulting lateral wall surfaces of a photopolymer structure assume a xe2x80x9clog cabinxe2x80x9d appearance exemplified by a series of parallel, horizontally disposed and clearly discrete (although mutually bonded) xe2x80x9clogsxe2x80x9d of photopolymer. The horizontal crevices between the photopolymer logs may attract dust, moisture and other contaminants, and the effective wall thickness at the crevices is undesirably reduced.
However, even if the beam power was to be somehow evenly distributed over the beam spot, the log cabin effect cannot readily be avoided because the effective exposure time is greatest at the center of the scanning path of the round scanning beam, i.e., where there is exposure to the full diameter of the beam.
In the present invention, a conventional STL laser system is modified so as to provide a beam spot of annular configuration comprising a higher beam power density in laterally exterior portions thereof The annular beam spot surrounds a central xe2x80x9cholexe2x80x9d portion with a lower (or even essentially zero) beam power density. As a result, during scanning of the beam over the surface of liquid photopolymer, the layer being formed receives higher exposure along its lateral edges, and lesser exposure in its central or inner portion, which is still adequately exposed as the annular beam spot moves laterally as the beam is scanned. The resulting, multilayer walls are thus effectively and smoothly cured and hardened without overexposure of the central portions scanned. The exposure of the central portion may be controlled by manipulation of the laser beam power to be only sufficient to initially form at least a semisolid xe2x80x9cskinxe2x80x9d of hardened polymer at the top of a given layer over unpolymerized material retained therebelow between two walls of at least semisolid photopolymer resulting from exposure to the annular beam spot. A subsequent full hardening step such as may be conducted in an oven after the photopolymer structure is formed accelerates the polymerization of any liquid polymer trapped within the structure and forms a single cohesive object with smooth side walls, i.e., without the aforementioned log cabin effect.
The laser system used to achieve the desired annular spot in an STL apparatus may be configured in several ways. In one embodiment, the laser system includes an unstable optical resonator whereby power flows outwardly from the optical axis of the resonator. The optical resonator may be a confocal resonator employing one concave mirror and one convex mirror. The convex mirror forms the output coupling device for providing diffraction output coupling around the periphery of the output mirror. The output beam is collimated and focused to an annulus surrounding a central xe2x80x9cholexe2x80x9d of significantly reduced radiation density. In another embodiment, the laser system comprises a nitrogen ring laser.
Use of the annular beam spot in an STL apparatus for fabricating structures in accordance with the present invention results in structures having uniformly polymerized, smooth side walls. The STL fabrication of miniature structures with enhanced dimensional precision and much reduced dimensional variability is enabled by use of an annular laser beam spot employed in accordance with the present invention.